This invention relates generally to methods for filtering fine particles from a liquid carrier and more specifically relates to an improved method for dewatering aqueous slurries of very fine particle size minerals, including industrial minerals such as kaolin clays, calcium carbonates, and the like, as well as slurries of fine particle size coal, metalliferous ores, and the like.
In the course of processing numerous minerals, a step of dewatering by filtration is commonly utilized. Such a requirement may be exemplified by considering the processing of crude industrial minerals such as kaolin clays and ground calcium carbonates, where the crude materials are commonly subjected to a series of steps intended to remove undesired impurities such as discolorants, so as to ultimately provide a refined product which is susceptible to the particular use for which it is intended. In the instance, for example, of kaolins, such beneficiation may in some instances be accomplished by dry processing techniques, as for example by air flotation. More commonly, however, especially where the impurities or contaminants to be removed are in some manner chemically combined or intimately associated with the mineral, wet processes of various types are employed. These include such well-known processes as froth flotation, reductive and oxidative bleaching, and high intensity magnetic separation. This is in addition to wet classification, which can also be regarded as a beneficiation step achieving some of the aforementioned objectives.
Thus, in a typical procedure wherein the initial starting material is a crude sedimentary kaolin clay, including from about 1-2% titania, the said crude may be subjected to a separation process to reduce the titania content to below 0.8% by weight. More generally, the titania will be reduced to the range of from about 0.2 to 0.8. Where a froth flotation process is used for such purposes, the clay may be initially blunged and conditioned by forming an aqueous alkaline dispersion of the clay (pH adjusted to about 7-10 with ammonium hydroxide). The dispersion may include as a deflocculating agent various compounds known to be useful for such purposes, such as sodium silicate. Other useful agents include a water-soluble salt of a polyacrylic acid or polymethacrylic acid preferably having an average molecular weight in the range of from about 500-10,000. Oleic acid or other collector agent is added during the conditioning process. Reference may be had to U.S. Pat. No. 3,974,067 for further details of flotation procedures which may be utilized. Further aspects of flotation treatment of the aforementioned type can be found in numerous places in the prior art, including in Cundy, U.S. Pat. No. 3,450,257, and in U.S. Pat. Nos. 2,990,958 and 3,138,550.
The purpose of the froth flotation in the foregoing sequence is to remove titania; and accordingly other techniques can be utilized in place of or to supplement flotation, including by passing the slurry in relatively dilute form and while the clay is dispersed (typically at about 30% solids) through a high intensity magnetic field, e.g. via a magnetic separator of the type disclosed in Marston, U.S. Pat. No. 3,627,678. Such device comprises a canister packed with stainless steel wool at which enveloping magnets are capable of providing a high intensity field of 12 kilogauss or higher. Froth flotation may be combined with magnetic separation to achieve additional effects, see for example Nott, U.S. Pat. No. 3,974,067.
A further commonly used method for improving the brightness and whiteness of kaolin clays involves chemical bleaching. In this connection, it is noted that one of the principal sources of discoloring contaminants in the crude clay takes the form of insoluble oxides of iron. Thus, a common bleaching technique for removing the said contaminants, involves forming the clay into an aqueous slurry, acidifying the slurry to a pH of the order of 3.0 to 4.0 and adding a slurry soluble salt of hydrosulfurous acid. The general objective of this operation is to provide the S.sub.2 O.sub.4= ion which acts as a reductive bleaching agent. Such ion functions to reduce the ferric compounds present in the slurry to ferrous form, the latter being readily soluble and therefore removable by subsequent washing, dewatering and filtering operations.
A still further type of beneficiation treatment applicable to kaolin crudes involves high temperature calcination. It is useful here to point out that those skilled in the art of kaolin processing and products draw a relatively sharp distinction between so-called calcined kaolins and kaolins which have not been subjected to calcination and are usually referred to as "hydrous" kaolins. With respect to such terminology, it is noted that the prior art literature, including numerous of the prior art patents relating to the field of kaolin products and processing, often uses the term "hydrous" to refer to a kaolin which has not been subjected to calcination--more specifically, which has not been subjected to temperatures above about 450.degree. C., which temperatures serve to impair the basic crystal structure of kaolin. These so-called "hydrous" clays may have been produced from crude kaolins, which have been subjected to beneficiation as, for example, to froth flotation, to magnetic separation, to mechanical delamination, grinding, or similar comminution, but not to the mentioned heating as would impair the crystal structure.
In an accurate technical sense, the description of these materials as "hydrous" is, however, incorrect. More specifically, there is no molecular water actually present in the kaolinite structure. Thus, although the structure can be (and often is) arbitrarily written in the form 2 H.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2, it is now well-known that kaolinite is an aluminum hydroxide silicate of approximate composition Al.sub.2 (OH).sub.4 Si.sub.2 O.sub.5 (which equates to the hydrated formula just cited). Once the kaolin is subjected to calcination, which, for the purposes of this specification means being subjected to heating of 450.degree. C. or higher for a period which eliminates the hydroxyl groups, the crystalline structure of the kaolinite is destroyed. Therefore, such material, having been thus calcined, cannot correctly be referred to as a "kaolin". Accordingly, it should be appreciated that henceforth in this specification, unless otherwise indicated, when the term "kaolin" or "kaolinite" is utilized without a qualification (such as "calcined kaolin"), such term necessarily implies that the original structure of the material is intact. Thus, the unqualified term "kaolin" as used herein, can be considered to be equivalent to the technically inaccurate (but oft-occurring) prior art usage, "hydrous kaolin" or sometimes simply "hydrous clay".
Detailed discussions of calcined clays and their method of preparation may be found in numerous prior art patents. Particular reference may be made in this connection to U.S. Pat. No. 3,014,836 to Proctor, Jr.; U.S. Pat. No. 3,586,523 to Fanselow et al; and to A. D. McConnell et al, U.S. Pat. No. 4,381,948. The procedures set forth for producing a calcined clay is detailed in the said McConnell et al patent and provides a product substantially corresponding to the commercially available product ALPHATEX.RTM. of the present assignee ECC America Inc. In the said procedure, which is exemplary of modern practice in the calcined clay art, the crude kaolin clay is blunged and dispersed to form an aqueous dispersion. The blunged and dispersed aqueous slurry is subjected to a particle size classification from which there is recovered a fine fraction slurry of the clay. Following this, the clay may be dewatered by being flocculated and filtered, redispersed as a slurry and dried; or the classified slurry may be dewatered by directly drying, for example by spray drying.
As is discussed in Fanselow et al and elsewhere, the calcined clay process as same has been outlined, can be supplemented by use of additional beneficiation steps such as those previously discussed, i.e. froth flotation, high intensity magnetic separation, and the like.
It will be evident from the foregoing that whether one is considering the processing of so-called hydrous clays or of calcined clays, at various points in the processing of same, dewatering by filtration is a common and necessary step. The predominant practice in the kaolin industry calls for such filtration to be accomplished by rotary vacuum filters (RVF), although other instrumentalities are also used as will be further discussed. Dewatering of kaolin slurries by use of such apparatus is normally accomplished in an acid (3.0-3.5 pH) flocculated condition. Most commonly, the dewatering process usually follows bleaching, and entails heating to approximately 130.degree. F. and filtering using a rotary vacuum drum filter. Typically this technique produces a 56-62% solids product cake, and is regarded as the industry standard.
Dewatering by use of plate and frame presses, is also accomplished in an acid (3.0-3.5 pH) flocculated low solids (20-30%) condition. This technique produces a 70-72% solids product cake, but is not generally considered cost effective. Automation in recent years has somewhat offset the economic disadvantage of this type of apparatus.
In a series of patents including e.g. U.S. Pat. Nos. 3,753,498, 3,753,499, and 3,782,554, assigned to ECC International Limited of St. Austell, Cornwall, England, tube presses are described, which are also useful when filtering kaolins and similar very fine particle size minerals. Dewatering is again accomplished in an acid (3.0-3.5 pH) flocculated low solids condition. The technique has the advantage of producing a 75% solids product cake, but is unfortunately maintenance and cost intensive.
One of the known exceptions to the inability of prior art filtering techniques to effectively filter slurries of dispersed very fine particle size minerals such as kaolins having particle size distribution (PSD) such that 50% or more of same by weight are of less than 0.5 .mu.m, involves use of the so-called electrically augmented vacuum filter ("EAVF"). Reference may be had in this connection to such use in Mixon, Jr., U.S. Pat. No. 4,246,039. Use of an EAVF enables filtering of dispersed (7.0-9.0 pH) 38-40% solids feed slurries; and the said filter is also capable of producing a 75-80% solids product cake. The EAVF technology is such that flocced kaolins cannot be effectively filtered. While it would be thought because of these advantages that the said filtering technique would have come into widespread commercial use, such is decidedly not the case. This is explained by the complexity of the device (which is based upon the principle of providing an electric field which appropriately interacts with a membrane), and the attendant high costs of maintaining and operating such device. Simply stated, the devices have not proved reliable or economically feasible for widespread application.
A further technique which has found application to dewatering of clays, is simple thermal evaporation. According to this technology, 60% solids filtered product can be thermally evaporated until the slurry reaches 70% solids. This technology is of specialized interest in application, but cannot meet the variety of needs and conditions for use required in industry such as the kaolin processing industry.
A still further technique for dewatering involves the use of centrifuges. These systems can be very effective for dewatering especially with feeds in the 20-30% range, and products up to the 50% range. When these limits are reached, the slurry viscosity and mechanical speed constraints, reduce cost effectiveness and performance to unacceptably low levels.
While the foregoing discussion has been especially directed at the unique aspects of beneficiation of kaolins, it may be noted that the requirement for dewatering of a fine particle size mineral slurry arises in many other environments. In recent years for example, precipitated calcium carbonates ("PCC's") have come into increasing use in paper manufacture. Both during production of such PCC's and in the course of handling and shipping same, it is often desired to prepare a high solids aqueous slurry or to increase the solids content of a given slurry. In either event, an effective filtration method is much sought after.
For some years the concept of ceramic filtration has been known as a method for dewatering a range of materials including slurries of particulate minerals, and such filtration techniques are in commercial use at various points in the world for the dewatering and filtering of relatively coarse minerals. Certain recent developments in the field of ceramic filtration have provided ceramic filter constructions which have proved very effective in the filtration of relatively coarse-sized minerals. Reference may especially be had in this connection to international applications WO88/06480, WO88/07402, and WO88/07887. The ceramic filtering elements described in these applications are characterized by an underlying porous support layer, and an overlying porous filtration layer. The ratio of the mean pore size in the support layer to that of the filtration layer is in the range of about 2 to 50. The pore size in the support layer is of the order of 4-50 .mu.m and that of the filtration layer is in the range of from about 0.5 to 3 .mu.m. Particularly in the filtration layer the pores are relatively uniform--i.e., there is a very narrow distribution (within the ranges indicated), so that one may operate at a determined bubble point with assurance that negligible air will pass through the filter during filtration, which may be accomplished at a very high efficiency. Thus in the filtration layer substantially all of the pores are in the 0.5 to 3 .mu.m range. Further details respecting composition and preparation of these ceramic filter materials are set forth in said publications, the entire contents of which are hereby incorporated by reference.
Heretofore, the use of these materials has been limited to relatively coarse particle minerals, having a size range of approximately 1 .mu.m or (in general) much higher. In the case of minerals such as kaolins, it has heretofore proved impractical on any commercial scale to successfully effect filtration of such materials where the average particle size is substantially below about 1 .mu.m, where substantial quantities of the particles are &lt; 1/2 .mu.m, and where the slurry from which said kaolin is to be filtered contains the kaolin is a dispersed or fully dispersed state. This is indeed the reason why the great bulk of filtration practiced in the kaolin industry, requires the use of flocculation as a precedent step to filtration. Consideration of the prior art and patents and the like will amply demonstrate this point.
Aside from the fact that flocculation introduces a complex and additional step into kaolin processing, it also has the effect of adding further undesired chemical species into the slurry, i.e., the flocculation is usually accomplished by acidification, e.g. with the addition of sulfuric acid or other acid-introducing species, which further complicates the process chemistry due to the need for later neutralization. The result of the latter can be the creation of fluxing agents, unless a great degree of washing is used, which indeed is one of the further consequences of flocculation and conventional filtering. (Fluxing agents are of great concern where the kaolin being processed is intended as a calciner feed.)
In accordance with the teaching of our U.S. Pat. No. 5,098,583, an improvement was provided which is applicable to the process for beneficiating a crude kaolin wherein a wet classification step provides a classified kaolin fraction having a PSD such that at least 50% by weight thereof are of less than 1/2 .mu.m ESD and substantially 100% by weight are of less than 10 .mu.m ESD; and wherein an aqueous slurry of the classified kaolin is at least partially dewatered by filtration. According to our said improvement, at least one or more of the filtration steps are effected by flowing the aqueous kaolin slurry through a ceramic filter plate. The filter plate is characterized by a porous support layer and an overlying porous filtration layer, the ratio between the respective mean pore sizes of the filtration layer material and the material of the base layer being between about 2 and 50. The pore size in the filtration layer is in the range of from about 1 to 3 .mu.m and the pore size in the base layer is in the range of from about 4 to 50 .mu.m. The flow through the filter plate is from the direction of the filtration layer toward the base layer, and is effected by establishing a fluid pressure differential across the filter plate, preferably by application of vacuum or partial vacuum conditions at the side of the filter plate to which the water is drawn. The kaolin subjected to the filtration may be substantially fully dispersed in the slurry being treated, and the pH of the slurry can be in the range of from about 6.0 to 8.0. The kaolin subjected to the filtration may also be in a flocculated state or in a state of partial dispersion. More generally, the pH of the slurry treated by the invention may therefore reside anywhere in the broad range of from about 2 to 10.
The method of U.S. Pat. No. 5,098,583 may be used to treat a grey kaolin where the classification step has provided a fraction having a PSD such that at least 95% by weight thereof are of less than 1 .mu.m ESD. This fraction can also have been subjected to a high intensity magnetic separation of a dispersed slurry and the output from the magnetic separator may be passed to the ceramic filter without use of any intervening flocculation step. Similarly, an oxidative bleaching step, as for example by use of ozone, may have been used upstream of the magnetic separation of the grey kaolin. The partially dewatered slurry from the ceramic filter can be spray dried to provide a feed for subsequent calcination (oxidative bleaching would not be used when the product was intended for calcination.
More generally, the method of our prior said patent may be used to treat any coarse or fine grey kaolin. These kaolins cannot normally be beneficiated by oxidative bleaching and also do not flocc well. The invention, however, facilitates beneficiation of these grey kaolins in a process which can include one or more of such steps as blunging, degriting, magnetic separation, ozonation, classification and dewatering.
In the case of the flocculated kaolin, the partially dewatered kaolin slurry from the ceramic filter can be dispersed and then dewatered at a further ceramic filter to provide a high solid slurry including &gt;70% by weight solids. This high solids slurry may be used directly in that form, or may be spray dried to provide a product. Where the kaolin in the slurry provided to the ceramic filter is at least partially dispersed, it may include about 60% solids, and the slurry can be further dewatered by the ceramic filter, to again provide a slurry having &gt;70% solid.
The method of our prior invention offers several benefits in kaolin processing. Among these are: Kaolin particle size ranging from coarse to ultrafine (10 .mu.m to 1/4 .mu.m) can be filtered. Kaolin processing can be effectively performed in a wide range of pH's, i.e., from 2 to 10. Economical dewatering rates and product solids using various feed solids 5-70%, solids with product solids up to 80% can be obtained using this technology. The process operates under broad temperature ranges, and filters flocced, semi-dispersed, and dispersed feeds, so that the resulting filtrate is clear with little or no suspended solids.
Both in the instances of kaolin filtration as taught in our U.S. Pat. No. 5,098,583 patent, and as well in the instance of other prior disclosed use of ceramic filters of the types discussed herein, serious impediments to practical large scale commercial use of same has been presented as a consequence of the tendency of such filters to become clogged with the fine particles of the slurry being filtered. Directly related to this is the difficulty encountered in removing the filter cakes, that having been formed upon the surface of the filter via a suction process, is found to adhere so tenaciously to such surface, that a scraper or doctor blade or other mechanical attack on the filter cake is conventionally required to effect removal of the cake. See in this connection WO 88/07887. These prior art techniques of cake removal are at best inefficient, of limited efficacy, and are inconsistent and relatively unpredictable in results. They are, furthermore, poorly adopted to an operation predicated on a continuous or semi-continuous operation; i.e. one in which the filter plate is rapidly used for filtration, and then is subjected to cake removal, after which the plate must be fully ready for further recycling in the same process.
In our U.S. patent application Ser. No. 824,032, filed Jan. 22, 1992, the disclosure of which is incorporated herein by reference, a method is disclosed based upon use of ceramic filter plates of the type discussed above, which obviates many of the difficulties encountered in cake removal. However, ceramic plates despite some very noteworthy advantages in the present environment, have certain inherent problems such as brittleness and strength under load. Accordingly, structures which could perform effectively in the environment of the Ser. No. 824,032 invention while obviating the cited disadvantage have been sought after, and in accordance with the present invention have now been found.